Why is this NOT a sync issue

@johnrellis: You got the quiz answer correct. 

However, you missed the bonus points.   You can do the same calculation you proposed using Lightroom.  Where in LIghtroom is the feature that enables you to do this?

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One cause of a discrepancy is that videos synced from LR Cloud to LR Classic won't appear in All Synced Photos.  All Synced Photos contains files that are synced in both directions, but since LR Classic won't sync videos back to LR Cloud, videos won't appear in it.

The easiest way to find such videos is by using the Any Filter plugin:

Without the plugin, you could do this procedure:

1. In LR Cloud, create an album "All Cloud Photos" and put every photo and video in it.

2. Wait until it syncs with LR Classic. The LR Classic collection All Cloud Photos will contain both photos and videos from LR Cloud.

3. Subtract the photos in All Synced Photos from the collection All Cloud Photos -- the remaining files will be videos.  The subtraction can be done in just a few steps and is left as an exercise for the reader.